zfs-list.8: clarify listing snapshots

[behlendorf]

Motivation and Context

Issue #11562.

cc: @ThePythonicCow

Description

Clarify how to include snapshots in the zpool list outpool by
referencing the full name of the listsnapshots pool property, and
the zpool list -t snapshot option.

How Has This Been Tested?

Proofread via man/man8/zfs-list.8, snippet:

   If specified, you can list property information by the absolute path‐
   name or the relative pathname.  By default, all file systems and vol‐
   umes are displayed.  Snapshots are displayed if the listsnapshots pool
   property is on (the default is off), or if the -t snapshot or -t all
   options are specified.  The following fields are displayed: name, used,
   available, referenced, mountpoint.

[updated]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.